The present invention provides a method for compensating errors due to the Goos-Hanchen effect in an autofocus (AF) system.
The Goos-Hanchen (GH) effect produces a shift of a beam when incident on an optical interface (e.g. a substrate that is imaged by an imaging optical system in the production of a semiconductor wafer). In one way of looking at this effect, any monochromatic beam incident on a reflecting surface can be decomposed into a sum of plane waves. The reflecting surface (e.g. the substrate surface) then produces a different phase for each plane wave depending on its angle of incidence. Very often, over a small range of angles, this phase on reflection will either increase or decrease with the angle of incidence producing a tilted wavefront in the far field, which is the same as a shifted spot at the reflecting surface—the near field. In an imaging optical system that includes a reflective surface near an image, this effect will produce a shift of the image. This is also true in an autofocus system that images some source object (e.g. a slit or fringes) onto the surface of investigation (e.g. a wafer) at a glancing angle of incidence and then relays that image to a detector. The position of the image on the detector will depend on the height of the surface of investigation, but will also depend on the variation of phase on reflection produced by that surface—the GH effect. In an AF system, this means that variations in the surface construction, which may consist of many thin film layers and printed circuit patterns, will produce an error in the surface height measurement; we call this the GH error.
The problem with the GH error is that it can vary with underlying substrate patterns, and coating thicknesses, and that variation can be large, e.g. several hundred nanometers to several microns. Moreover, that variation is typically indistinguishable from the substrate (substrate) topography in an optically based AF system.
One approach to compensating the GH-effect is to use ellipsometry to determine the substrate film structure, and then use the film structure to estimate the GH error, and finally subtract that error from the measured surface height. However, ellipsometry requires a complex optical system of its own, a big increase in computational power, and a lot of input from the user.